Broad bandwidth optical transmission systems have received a great deal of attention in recent years. Such systems require broad bandwidth optical amplifiers to achieve transmission of high capacity wavelength division multiplexed signals. A type of optical amplifier that is sometimes employed is a so-called distributed amplifier in which signal amplification occurs along the signal transmission path. Examples of distributed amplifiers include distributed Erbium doped fiber amplifiers and Raman amplifiers.
Raman amplification is accomplished by introducing the signal and pump energies along the same optical fiber. A Raman amplifier uses stimulated Raman scattering, which occurs in silica fibers (and other materials) when an intense pump beam propagates through it. Stimulated Raman scattering is an inelastic scattering process in which an incident pump photon looses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations (i.e., optical phonons). That is, pump energy of a given wavelength amplifies a signal at a longer wavelength. The relationship between the pump energy and the Raman gain for a silica fiber is shown in FIG. 1. The particular wavelength of the pump energy that is used in this example is denoted by reference numeral 1. As shown, the gain spectrum 2 for this particular pump wavelength is shifted in wavelength with respect to the pump wavelength.
The pump energy may be introduced on the transmission fiber so that the pump and signal are either copropagating or counterpropagating with respect to one another. Additionally, in yet another pumping arrangement, both copropagating and contrapropagating pump energy may be employed, which will hereinafter be referred to as bidirectional pumping. Bidirectional pumping offers a number of advantages over unidirectional pumping (whether copropagating or contrapropagating). For example, bidirectional pumping may advantageously provide a substantially constant distribution of pump power over the transmission fiber, reduce the impairments caused by double Rayleigh reflections, and improve the noise performance. In contrast, when unidirectional pumping is employed, the distribution of pump power over the transmission fiber decreases at least exponentially (depending on pump depletion) from the point along the transmission fiber at which the pump power is introduced.
It has been found that double Rayleigh scattering can be a serious noise source in typical distributed fiber amplifiers, due largely to the relatively long length of the typical fiber. Double Rayleigh light is twice backscattered by unavoidable density fluctuations in the amplifier fiber such that it propagates in the downstream direction (i.e., the direction in which the signal travels) and adds noise, specifically, multiple signal interference, to the signal. Although the amplitude of double Rayleigh light is generally small, the light is amplified in the same way the signal is amplified in the Raman amplifier, resulting in significant noise amplitude. Indeed, it can be shown that double Rayleigh light increases as the square of the length of the amplifier fiber for an ideal distributed amplifier. Thus, noise due to this mechanism can be several orders of magnitude more in Raman amplifiers and in distributed erbium doped fiber amplifiers than in lumped erbium-doped fiber amplifiers. Bidirectional pumping reduces the imparments caused by this effect.
One problem that arises in forward and bidirectionally pumped Raman amplifiers is that a Raman amplifier employing a copropagating pump beam is generally noisy. This effect is well known to be caused by coupling of pump intensity fluctuations to the signal. It has until now necessitated backward pumping of Raman amplifiers.
Thus, while a bidirectionally pumped Raman amplifier has a number of attractive features, a significant disadvantage is the relatively high level of noise produced as a result of the contribution from the copropagating pump beam.